This invention relates generally to the field of photoreproduction and deals more particularly with an improved boxlight for a projection type photoreproduction system.
U.S. Pat. No. 3,998,546 and U.S. Pat. No. 4,582,406 disclose photoreproduction systems in which high quality photographs are produced by projection photography involving the projection of light onto photosensitive film through an image on a film transparency. The light emanates from high intensity lamps which are housed in a light box known in the industry as a boxlight. This type of process is commonly used in color photography where the transparency is a screened color separation containing halftone dots.
Photoreproduction equipment of the type disclosed in the above referenced patents has been commercially successful and has in fact nearly largely supplanted the prior practice which involved hand stripping halftone separations and other elements together prior to printing of a composite color print. However, prior photoreproduction machines of this type are not wholly free of problems, and the problems that remain are of particular concern in extremely high quality halftone work such as that used in the printing of advertisements in mass circulation periodicals. One of the most troublesome problems is that of light falloff near the extremities of the image plane. Although this problem is complicated from an optical standpoint and disagreement remains as to all of the factors which cause it, there is general agreement that the optical properties of lenses and an effect commonly referred to as "cosine fourth" losses are contributing factors.
Virtually all lenses provide the sharpest detail in the center of the image area near the optical axis of the lens. Because light intensity varies inversely with the square of the distance it travels and because the distance light travels in an optical system increases as it moves farther off axis, the falloff in intensity increases rapidly toward the edges of the image plane. In addition, a specified unit of the film located on the optical axis "sees" a circular iris as a circle while an off axis film unit "sees" the same iris as an oval. The oval becomes more narrow as the distance from the optical axis increases, and the areas near the perimeter of the film image are thus exposed to less light because of this factor also.
The cosine fourth effect is more subtle but has long been known to optical physicists. This phenomenon gets its name from the fact that the illumination of a point on the image plane varies with the fourth power of the cosine of the angle between the optical axis of the lens and the chief light ray. Since the cosine decreases as the angle increases, the larger angles of rays located far off axis results in relatively large cosine fourth losses near the periphery of the image plane. Consequently, it is evident that lens resolution and cosine fourth phenomena are additive and result in the most serious problems near the perimeter, since both are most pronounced near the peripheral areas of the image plane.
State of the art techniques in lens design and manufacture can produce high quality lenses that are capable of at least in large part compensating for the falloff that is caused by the lens. However, this is not accomplished without introducing unusual aspects such as requiring operation in a prescribed narrow spectral band for which the lens is designed, excessive lens weight (upwards of 30 lbs.), and high lens cost ($10,000 or more per lens). Even then, the cosine fourth losses remain and can create unacceptable quality problems in the finished print.
Any quality reduction at all is of great concern in modern day halftone work which must be of the highest quality. Examples are the double page advertising spreads often found in mass circulation magazines or books and any color halftone work as large as 11.times.17" or larger. In this type of high quality work, even the slightest falloff in the illumination at the ends or corners (or undue light concentration at the center or elsewhere) renders the optical system incapable of recording halftone dots at the two ends of the gray scale (the highlights and shadows of the picture).
In the manufacture of "flats" (large formats containing many pages ready to plate for large-press printing), the common expectation is to hold 1% to 99% dots of the standard 150 line screen count. These dots in a 150 line screen are only 0.000625 inch in diameter, so it is easy to appreciate the difficulty of retaining them in a second generation film. Cosine fourth losses and other problems in the illumination provided by the boxlight can make it impossible to meet the industry expectation, even if the best available lenses are used.
Even though cosine fourth losses have been recognized in projection as well as camera work, there have been few approaches made to the problem that have been successful in eliminating it. Solutions that have been proposed for use in other types of optical systems such as document photocopying (see Fowler U.S. Pat. No. 3,669,538 and Critchlow et al. U.S. Pat. No. 4,298,275) are unsuited for projection photography in large format printing and are in any event of questionable effectiveness in solving the problem of cosine fourth losses.
Such proposals as those suggested in the aforementioned patents would be unsuited for use in projection photoreproduction for myriad reasons. For example, the transparency must be oriented with its long axis horizontal in some cases and vertical in other cases, depending on the ultimate sheet size, the layout of the printed piece, and the number of pages on the "flat". An eight page flat made up of 81/2.times.11 inch pages may measure as little as 22.times.34 inches if the pages are to be cut apart or as little as 23.times.35 inches if the pages are to be folded up into a "signature". In either case, the arrangement of the pages is two rows disposed one above the other with each row including four pages and each page oriented vertically. This is the arrangement shown in FIG. 7. However, if two of the adjacent pages are ultimately to be used as a double page spread, they must be photographed horizontally (as exemplified by "Exposures 5 and 6" in FIG. 7). On the other hand, if that same double page spread is to be part of a 16-page flat, all of the 81/2.times.11 inch pages on the flat will be horizontally composed, and the double pages 11.times.17 inch area which is to be photographed as an entity must be filmed in the projector with the original transparency held vertically on the subject holder. This is illustrated in FIG. 8, with Exposures 3 and 6 constituting the double page spread.
As another example, the 81/2.times.11 inch or 9.times.12 inch pages in a 32 page flat are composed vertically in rows (see FIG. 9), while the pages in a 64 page flat are composed horizontally. Again, the lighting system is required to accommodate both horizontal and vertical orientations of rectangular transparencies. Other situations may require the transparency to have a 45 degree attitude or some other skewed orientation.
For these reasons, a lighting system which is specifically designed in a shape that accommodates only horizontal or only vertical attitudes (or any other fixed attitude) is unacceptable for use in multiple image step-and-repeat photography which uniquely requires that all attitudes be handled. Accordingly, light falloff that is accentuated in any direction cannot be tolerated because the lighting effect in that direction would be lacking and would detract from the quality of the finished product when its long axis must be oriented in the direction of maximum light falloff. Stated another way, the light distribution should be symmetrical about the optical axis of the photographic system.